Questions tagged [lift]

The force acting on an aircraft in opposition to gravity which keeps the aircraft in the air.

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90
votes
14answers
16k views

Are wings any more efficient at creating lift, versus orienting the engine's thrust downwards?

As I understood from this article, wings on an airplane basically create lift by pushing air down. Maintaining the lift force expends energy because air needs to be continually accelerated downwards. ...
56
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11answers
33k views

How do wings generate lift?

Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?
44
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9answers
10k views

How complete is our understanding of lift?

I'm currently studying for my PPL and one of the accepted textbooks contains the following disclaimer at the end of the Principles of Flight section on lift: It is important to note that the ...
44
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3answers
7k views

Was the Space Shuttle aerodynamically neutral while piggybacking, or did the combination act like a giant biplane?

Trying to investigate this question, I see that 'Ask the Captain' says: The weight of the shuttle is calculated like any payload. The 747 produces enough lift to fly and to carry the weight of ...
38
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4answers
12k views

How do conventional missiles fly?

With the exception of cruise missiles, most missiles don't have any obvious lifting device. If you watch a missile being fired, prior to its rocket firing it does exactly what you'd expect: ...
36
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2answers
4k views

Why do helicopters not roll over when flying forwards?

Related to this answer: with velocities so different at forward going and rearward going blade, why does the helicopter not roll over? The higher airspeed at the forward going blade should cause more ...
30
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4answers
6k views

Principle of aerodynamic lift: are misconceptions also taught in flight schools?

Describing lift as the result of "equal transit time" on both sides of an airfoil is a fallacious theory widely found in technical books and articles for general public (see below for details about ...
29
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8answers
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Is excess lift or excess power needed for a climb?

As answered in this question, aircraft need excess power - not excess lift - to climb. This is plausible when the aircraft's thrust vector has a vertical component (its nose and engine points upwards),...
28
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2answers
15k views

What is vortex lift?

In a recent question I asked, I was given an answer by Peter Kämpf and he described something about lift being created by a vortex used on delta wings and the Bird of Prey wing. How exactly does ...
27
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2answers
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How exactly can an airship lose lift?

On this wiki page (under 'Comparison with heavier-than-air aircraft'), the altitude record of the L-55 Airship states: The ultimate altitude record for a rigid airship was set in 1917 by the L-55 ...
26
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8answers
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Does a sudden drop of the velocity of the headwind affect the airspeed by lowering it?

I had a discussion about air speeds with one of my instructors recently. He is claiming that there is a sudden drop in the wind velocity as you get closer to the ground (which is absolutely true, due ...
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5answers
11k views

Is there a maximum lift to drag ratio?

If I recall correctly, the best competition gliders have a L/D ratio as high as 60:1. What imposes this limit? Is there a maximum theoretical L/D ratio, or could sufficiently advanced materials allow ...
22
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4answers
3k views

Would an aircraft with contra-rotating propellers longer than the plane's wingspan be able to fly?

I enjoy taking the time to try and visualize how fictional aircraft might actually work in reality. Unfortunately there are some that seem highly impractical from an engineering student's point of ...
19
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4answers
5k views

Why is lift larger than thrust?

It's been a while since I tried to read a little about flight mechanics and aerodynamics, but I remember one question I had back then: Is the thrust of a common passenger aircraft (A320, B737) ...
18
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2answers
3k views

Why did the Solar Impulse flight take so long?

The Solar Impulse just made big news for flying from Japan to Hawaii on a solar-powered plane in five days. But five days seems really long. Let's do the math on that. The distance from Japan to ...
17
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7answers
16k views

Does lift equal weight in a climb?

This subject keeps coming up in the discussions and questions such as this one, which asks if lift equals weight in level flight. Good answers there, pointing out that upwards force has many sources. ...
17
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4answers
2k views

Why do planes not fly “backwards” (up from the planes POV) when in a vertical climb?

As suggested by a user in this question, I'd like to ask this question: One of the most basic aerobatic maneuvers is a vertical climb. This can be achieved by just pitching up to what appears to be ...
17
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1answer
2k views

How do disks generate lift?

Disks and disk-like objects (frisbees, pie pans, round cardboard piece, hats of all sorts, and many other things) can fly well when thrown along with a spinning force which creates a gyroscopic effect ...
15
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6answers
3k views

Is lift in fact a kind of drag?

Given the similarities between Lift and drag and their very similar relationships to the density, surface area, and airflow velocity, could one, in theory, describe aerodynamic lift as a kind of drag ...
15
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2answers
2k views

What phenomenon causes the increase of drag with rain?

I know that rain generally worsens the aerodynamic propeties of an aircraft. I'm wondering what processes cause the increase in drag. I'm mostly interested in General Aviation, thus relatively low ...
14
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7answers
7k views

Why does a stall decrease lift, rather than increasing it?

A stall occurs when the angle of attack of a wing or other airfoil becomes so high that the airflow over the upper surface of the wing separates from the wing, rather than remaining attached to it; ...
14
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3answers
15k views

How much thrust is needed by an aircraft to have vertical takeoff?

In some airshows, I have seen some big aircraft (like 737) takeoff almost vertical. Generally, fully-loaded passenger/cargo planes don't do that, I guess due to safety issues but I think there needs ...
14
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1answer
542 views

What amount of lift would be created in the fallacious “lift from equal transit time” theory?

Context A wrong --but simple to understand-- theory for lift states: Air molecules that were close before being separated at the leading edge must be close again after they join back at the trailing ...
13
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5answers
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Why are wings load tested upside down?

Why are wings load tested by putting weights on the underside of the wing (placed upside down)? A wing is being pulled into the air from its upper surface in flight, so shouldn't it be load tested in ...
13
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4answers
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What is the aerodynamic motivation behind vortex generators?

An airplane's wing creates lift efficiently when the airflow is parallel to the chord of the wing. Vortex generators twist the airstream on the wing surface. Doesn't this kind of twisted airflow ...
13
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2answers
26k views

What is the method to calculate a finite wing's lift from its sectional airfoil shape?

I am struggling to get my head around a concept that I believe should be fairly simple to understand. Lift versus drag and AoA data of many airfoils are freely available, for instance the NACA 4-...
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2answers
6k views

What is the actual air speed over and under a wing due to Bernoulli's Principle?

Bernoulli's Principle states that as a fluid speed increases, its pressure decreases, and vice versa. Air flowing over the wing of an aircraft flows faster than its neighboring air flowing under the ...
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5answers
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Could propeller wash provide sufficient lift to take off - even in theory?

I know this seems a crazy thing to ask but it is not the same as lifting oneself by the bootstraps. If you try to lift yourself that way, there are two equal and opposite forces, yet there are ...
12
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2answers
2k views

Why do the outer parts of wings generate less lift?

In this Wikipedia page it says: Constant chord: parallel leading & trailing edges. Simplest to make, and common where low cost is important, e.g. in the Piper J-3 Cub but inefficient as the ...
11
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3answers
2k views

What are the physical laws for upscaling an RC model to 1:1?

What are the laws of physics for upscaling an RC model from a smaller to larger scale or even to a full sized 1:1 aircraft? I guess the basic thing may be lift/weight ratio which was nicely described ...
11
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3answers
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How is lift provided by a wing affected by propeller wake?

Does the lift of that section of the wing which is in the wake of the propeller increase (because the airspeed in that section is higher)? If yes, is this fact used actively to improve the lift ...
11
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1answer
18k views

How to calculate the Lift Coefficient for the A320?

I'm trying to figure out the lift coefficient of an A320. What I'm doing is the following: during cruise, the lift is equal to the weight of the aircraft ($mg$) so that I used the cruise table taken ...
10
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4answers
8k views

Do any aircraft out there use negative lift?

I think negative lift is where high pressure forms on the top of the wing instead of the bottom. I know that Indy cars use negative lift and some sports cars do, but do any aircraft exist that use ...
10
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3answers
10k views

What is the lift force per unit span?

I got Aircraft Design: A conceptual approach for Christmas, and I'm having a hard time with lift coefficients because I honestly have no idea what "lift force per unit span" means, so can someone ...
10
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3answers
4k views

What is the physical meaning of circulation found in Kutta condition?

The Kutta–Joukowski theorem is applicable for 2D lift calculation as soon as the Kutta condition is verified. When this is the case, there is a circulation $\small \Gamma$ around the airfoil. My ...
10
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3answers
617 views

Why don't smaller powered airplanes have better lift-to-drag ratios?

Many airliners have LDRs near 20, and it sounds like the electric Eviation Alice may have an LDR of 24 at 240 kts, but many small planes typically have LDRs of only 8-10. It seems like they could ...
9
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3answers
990 views

How did the F28 have such a good L/D ratio?

The second link given on a Google search for "Breguet Equation" is to a PDF document from MIT. On page 7 of this document is a chart showing L/D ratios for various commercial aircraft (turboprop, ...
9
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1answer
738 views

Which books should I study to understand more about Supercritical airfoils?

I want to understand the theory behind supercritical airfoils and all the details about them. It would be nice if I could get a standard supercritical airfoil, its experimental Cl, Cd data in the ...
9
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5answers
890 views

Do most commercial jets have a negative lifting body?

I took a look at some example airfoils and noticed they all seem to have a downward hook at the end. I believe this draws the air downwards, creating a reaction force upwards: lift. Now here is a ...
9
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1answer
8k views

Does lift coefficient vary with the wind velocity for a given angle of attack?

I have this question in my mind. I have been trying to look for this in different books about aerodynamics and I couldn't find the answer. I know I could check the answer experimentally, if I had a ...
9
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2answers
874 views

How do helicopters handle P-Factor?

Propeller-powered aircraft have a "problem" called P-Factor, where the descending blade of the prop is generating more lift (a.k.a. thrust) then the ascending blade due to a higher angle of attack ...
9
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2answers
1k views

Why do delta wings generate less lift at a particular angle of attack compared to straight wings?

I simulated a delta wing and a straight wing in xflr5 with the same profile, wing area and airspeed (very low airspeed. around 10 m/s. it's for a model aircraft). The straight wing was operating at ...
9
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1answer
623 views

What is the best combination of airfoil shape for flight in ground effect over water?

What is the best airfoil shape for flight in ground effect over water?
8
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4answers
16k views

What are the differences between air brakes, spoilers and lift dumpers?

So many people use these words interchangeably, but after a little searching on Google, I found they are not quite the same thing. All the links I read explain things in a misleading way which pushed ...
8
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5answers
10k views

Where can I find data tables for lift and drag coefficients of airliners?

I'm teaching an introductory course in fluid dynamics and I'd like to show some "real" data regarding drag and lift forces on an airfoil. It is easy to find data on the geometry of typical airfoils, ...
8
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1answer
3k views

What is the aerodynamic centre and how does it relate to the pitching moment?

Okay so I have been trying to get my head around the Aerodynamic centre and the Pitching moment. Wikipedia really isn't helping much but from what I can gather the Pitching moment is a force caused by ...
8
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3answers
3k views

Is the low air pressure on top of the wing the major contributor to lift?

I am kind of old time flyer. Started to fly with those primitive hang-glides with titanium frame and the yoke control, around 1966 and then after a few close calls and getting older I switched to ...
8
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1answer
2k views

What part of the F-18's wing profile allows for high AoA flight?

I'm trying to analyse the relationship between the angle of attack and lift coefficient of a wing profile I tested. However, I am finding that the data shows that the wing hasn't stalled by the time ...
8
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1answer
593 views

Do software simulations and wind-tunnel tests accurately predict Cd and Cl in real-world conditions?

Will the lift coefficient (Cl) and drag coefficient (Cd) I obtain by using software simulations or subsonic wind-tunnel tests be the same as on an actual aircraft while cruising? Or will there be some ...
8
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2answers
327 views

Why is ground effect greater over smooth hard surfaces?

Helicopter Flying Handbook: Ground effect is the increased efficiency of the rotor system caused by interference of the airflow when near the ground. The air pressure or density is increased, ...

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